Aryloxylation

In a similar system, the reaction of the ferric(edta) complex with peroxycarboxylic acids was probed by adding 2,4,6-tri-fe/r-butyl phenol, ArOH.2 This experiment gave rise to the aryloxyl radical, ArO, which persisted for hours and was detected by its characteristic spectrum. It was indeed formed in the reaction mentioned, at a rate that was independent of [ArOH], It was proposed that ArO results from a reactive oxo-iron intermediate, tentatively (edta)FevO. 

Of these Meissenheimer-type reactions, classical C-chlorinations (Section 3.1.5), C-hydroxylations (Section 4.1.1), and C-aryloxylations (Section 4.4.1) have been covered previously. The remaining types are illustrated in the following examples. 

Ioki, Y., Aryloxyl Radicals by Photorearrangement of Nitrocompounds, J. Chem. Soc., Perkin Trans. II, 1240-f242 (f977). 

The oxidation of phenols forming aryloxyl radicals was first recognized by Pummerer and Frankfurter in 1914," who reacted 2,2 -dihydroxy-l,l -binaphthyl with ferricyanide, and isolated dimers and trimers of the radical 20 (equation 8), which was later observed by ESR." Many further examples were studied by Goldschmidt, Muller, and others," and an X-ray crystal structure confirmed dimer formation with C—O bonding at the 4-position for 3-bromo-2,4,6-triphenylphenoxyl. ... 

Substitution patterns on the B-rings appear to be the most important contributors. The 3, 4 -orthodihydroxy structure in the B rings confers the highest ability to the aryloxyl radical formed and participates in electron delocalization. A hydroxyl group at C-3 position is also beneficial to the... 

Only two azine aryloxyls have been described, the 2,4,6-triphenylpyridin-3-oxyl (111) and its pyrimidin-5-oxyl analog. ESR spectra have been published but not analyzed for these persistent species.364-367... 

An enantioselective aryloxylation of aldehydes is based on their prior conversion to an enamine through reaction with a chiral secondary amine catalyst. A subsequent inverse HDA reaction with o-quinones leads to 3-alkyl-2-hydroxy-l,4-benzodioxins with ee ca. 80% (Scheme 47). Manipulation allows the synthesis of (S)-2-alkyl-2,3-dihydro-l,4-benzodioxins <07TL1605>. In like manner, racemic nitidanin, which possesses antimalarial properties, has been synthesised through a regioselective cycloaddition of an o-quinone with a protected 3-arylpropen-l-ol <07TL771>. 

Bulky ligands as above have also proved to be effective in other palladium-catalyzed reactions of aryl halides, e.g., amination [16-19], Suzuki-Miyaura reaction [20-22], Mizoroki-Heck reaction [23, 24], Migita-Kosugi-Stille reaction [25], and aryloxylation and alkoxylation [26-28] as well as the reaction with various carbon nucleophiles as described below. The ligands are considered to enhance both the initial oxidative addition of aryl halides and the reductive elimination of products [29, 30]. The effectiveness of the commercially available simple ligand, P(f-Bu)3, was first described for the amination by Nishiyama et al. [16]. 

A low-resolution ESR spectrum has been observed of the aryloxyl 78, a product of the oxidative condensation of 2-naphthol. ... 

Addition of phenols to activated C—C multiple bonds is another method for O-aUcylation. Conjugated carbonyl compounds with -leaving groups react with metal phenoxides, giving the substituted products via addition-elimination, and the resulted /3-aryloxylated carbonyls are versatile intermediates for synthesis of heterocyclic compounds . Addition... 

The kinetics and mechanism of inhibition (inh) of free radical oxidation has been the subject of several earlier reviews . The main reactions for inhibited oxidation by phenols are outlined below. When a phenolic antioxidant is present, peroxyl radicals are trapped by H-atom abstraction from a phenolic hydroxyl group, followed by rapid recombination of peroxyl and resulting aryloxyl radicals (equations 10 and 11). 

The aryloxyl radicals formed in the initial antioxidant reaction of phenols (equation 1) may undergo several different kinds of secondary reactions, including Type (1), rapid combination (termination) with the initiating oxygen-centered radicals (equation 11) Type (2), self-reactions Type (3), initiation of new oxidation chains by H-atom abstraction from the substrate, the so-called prooxidant effect and Type (4), reduction or regeneration by other H-atom donors resulting in synergistic inhibition. The relative importance of these secondary reactions will be considered briefly here, since they may affect the overall efficiency of the antioxidant, which includes the antioxidant activity, as measured by the rate constant, (equation 10), and the number of radicals trapped, n. 

These researchers present a number of arguments and evidence, including large deuterium kinetic isotope effects, in support of a mechanism involving proton-tunneling in a charge transfer complex (equation 29), as the rate-determining step for the reaction of the hindered aryloxyl radical, ArO , with phenolic antioxidants and they propose that the mechanism applies equally well to attack by peroxyl radicals, R—O—O , on phenols. 

The hindered aryloxyl radicals may not model exactly the antioxidant mechanism of phenols with phenoxyl radicals. 

In certain media , even normally nmeactive aryloxyl radicals participate in these so-called pro-oxidant reactions dne to local high concentrations of ArO or certain physical restrictions which prevent their termination by radical-radical reactions. 

It is now generally accepted that the first step involves the formation of aryloxy-lead (IV) triacetates (1). These can decompose homolytically, leading to dimeric products via the aryloxyl radicals (2). They can also decompose heterolytically to cationic aryloxy species, which are then trapped by external nucleophiles. This has been exemplified by the reaction of phenols with lead tetraacetate in the presence of acetic acid or methanol. In the latter case, ortho-mcthoxy derivatives were formed. However, the preferential formation of 6-acetoxy-2,4-dienone derivatives is more likely explained by an intramolecular ligand coupling reaction (3) rather than by occurrence of a cationic... 

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